1. Field of the Invention
The present invention relates to an apparatus for detecting misfire in a multicylinder internal combustion engine and more particularly to an apparatus for detecting misfire, which checks, by detecting the change in the tone of the exhaust sound, whether there is misfire or not.
2. Description of the Prior Art
With an internal combustion engine having a catalyzer converter as an anti-pollution mechanism for exhaust gas in the intermediate portion of the exhaust passage, misfire in the cylinder produces unburnt gas, which is introduced into and often burns in the converter to damage the converter. In order to prevent the burning of the catalyzer converter, it is customary to drive a protecting device into operation by detecting a temperature rise in the converter. It, however, is more efficient to detect the occurrence of misfire earlier so that the protecting device may quickly respond to the accident or that the driver may be informed of the accident earlier.
As one of the conventional devices for detecting misfire is known a system which detects misfire by detecting the temperature of exhaust gas by thermoelectric transducers. With this system, it is necessary, to detect the change in the temperature of exhaust gas with high accuracy, to dispose the thermoelectric transducers in the exhaust passages in the vicinity of the respective exhaust ports of the cylinders. Accordingly, there must be provided a plurality of detecting elements, i.e. thermoelectric transducers, and the system must be so constructed as to be resistive to heat and therefore the system has a drawback of high cost. Furthermore, in case of detecting misfire by detecting the change in temperature, there is also a drawback that the response to the misfire is rather slow since only one occurrence of misfire cannot cause an appreciable temperature change but since it is only after several successive occurrences of misfire that the temperature of exhaust gas changes to a detectable degree.
On the other hand, the specification of the U.S. Pat. No. 3,540,262 discloses a system which acoustically detects the knocking and other abnormalities of the engine. According to the disclosed system, the electric signal obtained by detecting the exhaust sound by a microphone is merely displayed visually on, for example, a cathode ray tube. For this purpose, detector transducers for detecting the vibrations and the pressures due to knocking etc. must be provided at several points in or on the engine so as to detect the occurrence of knocking etc. so that the system becomes costly.